A current communications system mainly supports voice communication and data communication. Usually, a quantity of connections supported by a conventional base station is limited, and is easy to be implemented. However, a next-generation mobile communications system not only needs to support conventional voice communication and data communication, but also supports machine to machine (M2M) communication that is also referred to as machine type communication (MTC). According to prediction, by 2020, a quantity of MTC devices connected to networks may reach 50 to 100 billion, and this may far exceed a current quantity of connections.
For an MTC type service, because service types of the MTC type service differ from each other, network requirements differ greatly. Generally, there may be two services having the following requirements: One is a service that requires reliable transmission but is not sensitive to a delay, and the other is a service that requires a low delay and high-reliability transmission. It is relatively easy to process the service that requires reliable transmission but is not sensitive to a delay. However, for the service that requires a low delay and high-reliability transmission, if transmission is unreliable, retransmission is caused and an excessively large transmission delay is caused, and consequently, the requirements cannot be satisfied.
To handle a large quantity of MTC type services in a future network and satisfy low-delay and high-reliability service transmission, an uplink grant free transmission solution is proposed. In a grant free transmission system, there are a large quantity of terminal devices, but there are also an extremely small quantity of terminal devices that access a network, and the terminal device may randomly select a grant free transmission resource to send data. Currently, a network device needs to detect each pilot, to determine an active terminal device.
Currently, in a Long Term Evolution (LTE) system, an uplink pilot sequence includes a demodulation reference signal (DMRS) and a sounding reference signal (SRS). The DMRS is used for channel estimation, so that uplink data is demodulated according to a channel estimation result, and the SRS is used for uplink channel quality measurement and occupies an entire frequency band.
If the DMRS in the LTE system is used in an uplink grant free transmission system, the network device needs to detect every possible DMRS. A common DMRS detection method is: converting a received frequency domain signal to a time domain, to perform windowing and noise reduction processing. In this conversion process, inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) operations are required, and complexity is extremely high.
Therefore, to reduce pilot detection complexity becomes a technical problem urgently to be resolved in the grant free transmission system.